From:  Axel Hecht <l10n@mozilla.com>
Date:  10 Oct 2018 05:25:33 Hong Kong Time
Newsgroup:  news.mozilla.org/mozilla.dev.planning
Subject:  

Re: comm-central future as a branch

NNTP-Posting-Host:  79.206.189.209

Am 09.10.18 um 10:06 schrieb Magnus Melin:
> I'd like feedback on the below plan for having Thunderbird in a single 
> repository.
> Let's have follow-ups go to dev-planning
> 
> TL;DR version: planning to have current comm-central live as a branch 
> instead of a separate repository.

I like the general idea.

But. I'll need the Thunderbird team to provide the patches necessary for 
localization. That involves cross-channel/gecko-strings code generation, 
l10n.mozilla.org, the automation behind it, and pontoon.

I've tried to hard-code repo/default less often, but I'm pretty sure 
it's still in places. There's probably other work to be done, too.

The good news is, most of that stuff can be run locally in 
docker-compose or AWS, but it's not particularly easy to start with.

Regarding comm-central revisions, cross-channel references them like in 
https://hg.mozilla.org/l10n/gecko-strings/rev/290a9db4634e. No idea what 
other parts of the web link to comm-central revisions. I guess bugzilla 
commit comments?

Axel

> 
> ---
> 
> Background
> ----------------
> 
> When the Mozilla move from CVS to hg took place in 2008, the code 
> related to Thunderbird was not included in mozilla-central and instead 
> placed in the comm-central repository. Under the hood Thunderbird uses 
> the mozilla-central code, so to build it currently has to check out both 
> repositories. This is not ideal, for many many reasons.
> 
> I wrote [an email to 
> maildev](http://lists.thunderbird.net/pipermail/maildev_lists.thunderbird.net/2017-November/000875.html) 
> last year listing some of the drawbacks of this setup - but essentially 
> what it comes down to is that version control was not meant to be used 
> like this and it makes workflows error prone, in addition to having to 
> do various hacks in the build automation. Enabling autoland, phabricator 
> and other tools used by Mozilla core could also be done much more easily 
> for comm-central with a single repository.
> 
> The mail spurred some related discussion on git vs hg and importing of 
> history, but basically there was agreement among the comm-central 
> developers to go ahead with this change.
> 
> Thunderbird has now migrated its build automation to Taskcluster, and 
> also moved to building with mozilla-central as topdir, with a comm/ 
> sub-folder containing comm-central code. (We used to instead have a 
> mozilla subdir checkout of mozilla-central as child the comm-central 
> checkout.)
> 
> In 2014 there was [a proposal to merge comm-central code into 
> mozilla-central](https://groups.google.com/forum/#!msg/mozilla.dev.planning/P8oHKTCWSiQ/76ECituhK-IJ) 
> but this was rejected since the powers that be did not want 1) the 
> confusion about what code belongs to what project, 2) pull/tree size, 
> and 3) it was seen as irrelevant to Firefox. The proposal below has none 
> of these obstacles, and depending on the physical repository location, 
> may not change anything at all for a Firefox developer.
> 
> I think it's now time to revisit the single repository idea and pull 
> that through in one way or the other. So, on to the proposal. For 
> reference, on disk comm/ is on currently 122 MB (+ comm/.hg 192 MB), so 
> size-wise it is comparatively small, with a mozilla-central checkout 
> weighing in at 2.3 GB. Including the relevant CVS history, the comm/.hg 
> would be 317 MB in total.
> 
> How
> ------
> 
> To capture the essence of what Thunderbird is, we would make 
> comm-central a named branch of the mozilla-central. Let's call this 
> branch "comm/default".
> 
> Initially, we'd pull in a hg converted version of the current 
> comm-central default branch, putting all the files into a comm 
> sub-folder in the base directory of the comm/default branch. This is the 
> way that the Thunderbird build system already wants the code laid out. 
> With the comm sub-folder it will be clear to everyone involved which 
> code is on mozilla-central code and which is not. It is possible to 
> import the CVS history for the related directories too while we're at 
> it, but it adds some size. Importing related CVS history only adds 
> around 125 MB extra so I'm inclined to include it. With the new 
> comm/default branch pulled in, we merge the curent mozilla-central 
> default branch to comm/default. The resulting code is now the same as 
> status quo (of the both repos pulled in), just that it's all version 
> controlled in the same repository.
> 
> Once this is all set up, Thunderbird development work would move on on 
> the comm/default branch, and there would be periodical merges with the 
> (mozilla-central) default branch. These merges could be automated if 
> wanted.
> 
> Quite often it's the case that Thunderbird code needs to be adjusted to 
> build, due to changes in mozilla-central code. For cases of known 
> incoming bustage Thunderbird now would have the possibility of waiting 
> to do the merge until a fix is available. I would suggest never to back 
> out bustage causing mozilla-central changesets from the comm/default 
> branch, but to detect other incoming bustage by doing builds from a 
> comm/band-aid branch (branced from comm/default). The details on this 
> can be discussed later though, not to derail discussion about the main 
> issue here. There are many alternatives, and it's also completely 
> possible to do what we currently do.
> 
> Where
> ---------
> 
> In the earlier discussions i's become clear there is a bunch of 
> confusion around what is a branch and what is a repository, so please 
> note the difference. Aa repository is the physical location where the 
> history of a project is stored. Every repository does not have to carry 
> every branch. I.e., the mozilla-central repository could carry the 
> comm/default branch, or the comm/default branch could exist only in a 
> comm-central2 repository somewhere else. In all cases it's trivial for 
> developers who have mozilla-central checked out, to add the comm/default 
> branch to their local checkout.
> 
> So for the location there are at least three options:
> 
> A: branch in the mozilla-central repository
> B: branch in the mozilla-unified repository
> C: branch in a repository elsewhere
> 
> Not to disrupt operations too much I think it would be preferable to use 
> option B - create the branch in the mozilla-unified repository. It does 
> depend a bit on what plans Mozilla has for these repositories.
> 
> Creating the branch in mozilla-unified of course needs buy-in from the 
> Mozilla hg people, so please let me hear your opinions.
> 
> What about history?
> --------------------------
> 
> For the proposed approach (going through `hg convert`) commits are 
> preserved, but AFAIK it's not possible to preserve actual commit hashes. 
> `hg convert` will graft the original commits, and this adds the original 
> hash as an extra field to the new commit (use `hg log --debug` to see 
> it). I don't think this is such a big problem. The original commits 
> would be linked in the pushlog, the same way there is "converted from" 
> for mozilla-central-cvs, like 
> https://hg.mozilla.org/experimental/mozilla-central-cvs/rev/ab117d946959. I 
> don't see them e.g. in 
> https://hg.mozilla.org/users/mkmelin_iki.fi/thunderbird-central/rev/8f8b0933b611 
> so perhaps there is some server side feature that needs turning on?
> 
> Can I try it?
> ---------------
> 
> You can check 
> https://hg.mozilla.org/users/mkmelin_iki.fi/thunderbird-central/ which 
> carries the comm/default branch.
> 
> In your mozilla tree, pull in the branch like this:
> 
>       hg pull -u -b comm/default 
> https://hg.mozilla.org/users/mkmelin_iki.fi/thunderbird-central
> 
> To completely remove it again, use `hg strip "branch(comm/default)`
> 
> How did you do it?
> ------------------------
> 
>      CVS_GIT_REPO=~/tmp/mozilla-cvs-history/
>      CC_HG_REPO=~/Code/tb/mozilla/comm/
> 
>      git clone https://github.com/ehsan/mozilla-cvs-history.git 
> $CVS_GIT_REPO
> 
>      echo default comm/default | tee branchmap.txt
>      echo rename . comm | tee filemap.txt cc0.txt
> 
>      hg --cwd=$CC_HG_REPO up 0
>      (cd $CC_HG_REPO && ls -d */ | cut -f1 -d'/' | sed 's/^/include /') 
>  >> cc0.txt
> 
>      hg convert --filemap=cc0.txt --branchmap=branchmap.txt 
> $CVS_GIT_REPO single-repo/
> 
>      CVS_TIP=`hg id --cwd=single-repo -i -r tip --debug`
>      CC_0=`hg id --cwd=$CC_HG_REPO -i -r 0 --debug`
>      echo $CC_0 $CVS_TIP | tee splicemap.txt
>      # e4f4569d451a5e0d12a6aa33ebd916f979dd8faa 
> 93dd94bad983ce2eff5f228719c349b78a8a7f7d
> 
>      hg convert --config convert.hg.saverev=True 
> --splicemap=splicemap.txt --filemap=filemap.txt 
> --branchmap=branchmap.txt $CC_HG_REPO single-repo/
> 
> For fun, see all the history down to 1999 is there, like
> 
>      hg log comm/mailnews/mime/src/mimei.cpp
> 
> Then go on and add the default branch from mozilla-central and do the merge
> 
>      cd single-repo
>      hg pull -f -b default $CC_HG_REPO/..
> 
>      hg up comm/default
>      hg merge default && hg commit -m "Merge default to comm/default 
> branch"
> 
>      hg push -f -b comm/default --new-branch thunderbird-central-push
> 
> ---
> 
> Thoughts, comments, feedback appreciated!
> 
>   -Magnus
>